q3 - aimhigh

[ArchivalUser]

Addition of which of the following exhaustively 14C labeled substrates would lead to evolution of 14CO2
from a
cell-free suspension containing all the enzymes and substrates required for the synthesis of uridylic
acid?
A. Aspartate
B. Carbamoyl phosphate
C. Glutamine
D. Glycine
E. N10-Formyltetrahydrofolate

[ArchivalUser]

aa

[ArchivalUser]

aa

[ArchivalUser]

The correct answer is A. In the first step of pyrimidine synthesis, carbamoyl phosphate condenses with
aspartate
to form carbamoyl aspartate, in a reaction catalyzed by aspartate transcarbamoylase. In subsequent
steps, ring
closure occurs with the loss of water, followed by oxidation to yield orotic acid. Addition of ribose-5-
phosphate
produces orotidylic acid, which is decarboxylated by orotidylate decarboxylase to yield uridylic acid.
The carbon
dioxide that is evolved is derived from the alpha carboxyl group of aspartate.